This invention relates generally to devices and methods for de-marking packaged integrated circuits (ICs) and, more specifically, to systems and processes for removing a thin layer of filled polymeric or ceramic packaging material from surfaces of IC packages to produce a de-marked, highly reflective surface suitable for re-marking.
Since the first packaged integrated circuits (ICs) became commercially available, manufacturers have often found it necessary to identify packaged ICs by marking each IC or packaged assembly of ICs with the manufacturer's name, a part or serial number, or other identifying information such as a lot number or a wafer and/or die location. As the majority of ICs are packaged in a transfer-molded, filled polymer compound, most current marking systems have been developed for this type of packaging.
Manufacturers initially marked packaged ICs using mechanical ink transferring devices, such as stamps or rollers, with or without stencils, to transfer ink to the surface of an IC. Because of the mechanical nature of the process and the drying time associated with ink, ink stamping systems are relatively slow and the applied ink susceptible to smudging. Also, the quality of ink-stamped marks on packaged ICs can vary substantially over time and from IC to IC due to variations in the quality and quantity of ink applied, ambient temperature and humidity, and the condition and finish of the surface of the stamp and package.
Because of the deficiencies associated with ink stamping, manufacturers have in recent years switched to using a laser beam to mark the surface of a packaged IC. Unlike ink stamping, laser marking is very fast, requires no curing time, produces a consistently high quality mark, and can take place at any point in the manufacturing process.
Various machines and methods have been developed for marking ICs with a laser. As illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., and U.S. Pat. No. 4,375,025 to Carlson, a semiconductor device is placed in a position where a laser beam, such as that produced by a carbon dioxide or neodymium-yttrium-aluminum garnet laser, inscribes various characters or other information on a surface of the semiconductor device. The laser beam burns away a small amount of the surface of the semiconductor device so that the area where the characters are to appear has a different reflectivity from the rest of the surface of the device. By holding the semiconductor device at a proper angle to a light source, the characters inscribed on the device by the laser can be read.
U.S. patent application Ser. No. 08/590,919 filed Jan. 24, 1996, by one of the present inventors, assigned to the assignee of the present invention and hereby incorporated by reference, discloses yet another laser marking system which is operable at high throughput volumes and makes substantially constant use of a marking laser by use of a multi-track IC feed, marking and inspection procedure.
Because a laser mark is actually formed (burned) into the surface of the packaging material of a packaged device, a laser mark is a more permanent means of marking than ink stamping. If necessary, ink marks may be removed by heat or abrasion or, more typically, by employing solvents that will dissolve the ink and allow the ink to be removed from the surface of the IC without removing any measurable depth of packaging material from the surface of the IC. Conversely, when a recessed or "engraved" laser mark needs to be removed, a small layer of the surface of the package must be removed.
As described in U.S. Pat. No. 5,348,033 to Levit, it is often necessary to remove an existing mark on the package surface of an electronic component by employing some means of abrasion. For example, an existing mark on an IC must be removed when the IC is mismarked, when the quality of the mark on the IC does not meet acceptable parameters, or when, subsequent to marking, the IC is reclassified. After mark removal, the IC can be remarked with the desired information.
De-marking techniques currently known in the art include a Scotch-Brite.TM. belt surface finishing system and micro-abrasive blasting. Scotch-Brite.TM. surface finishing employs a mildly abrasive belt to buff and shine a surface of an IC for remarking after prior removal of an ink mark (typically by a solvent wash) but does not remove packaging material of the IC to any great extent, and is thus unsuitable for removing laser marks. The micro-abrasive blasting process is a grit blasting operation in which a grit-like material, such as a garnet powder or aluminum oxide particles, is directed onto the surface of an IC to remove ink markings, and the technique has also been attempted to remove laser marks.
Micro-abrasive blasting tends to remove a substantially uniform layer of material from the surface of an IC. Accordingly, cavities or depressions in the surface of an IC caused by an original laser marking operation are not easily removed by subsequent laser mark removal attempts using by micro-abrasive blasting. As a result, to produce a flat surface for remarking, an unreasonable depth of packaging material must often be removed, compromising the integrity of the packaged IC. In addition, particulate abrasive de-marking processes dull the surface finish of an IC, and any attempt to re-mark the ICs with a laser may not produce a visibly distinct mark because of the reduced contrast between the surface finish of the de-marked IC and the new mark. Accordingly, such de-marked ICs may need to be coated with ink before being re-marked by a laser to make the new laser mark easily visible. The current cost of such coating for re-marking approaches or exceeds eight cents per IC, so potential savings by eliminating the ink coating before re-marking are enormous, at thousands of ICs per day throughput.
Thus, it would be advantageous to provide an apparatus and method for removing laser marks from the surfaces of packaged ICs that produce a somewhat reflective surface finish suitable for re-marking with a laser.